Collaborators can stop the owner's agent run with agent access off #497

Dogfooding: a teammate (Luke) stopped my agent's run in my channel while Collaborator agent access was off.

Expected: with agent access off, only the owner can start runs, so only the owner (or the agent's own tooling) should be able to stop them. A collaborator should get the same refusal as invoking ("Only the owner can invoke agents here").

Cause: in packages/channel/src/channel.ts, #handle's stop case aborts the chat's run without any author check, and the stop request (packages/channel/src/protocol.ts) carries no author. ask and answer go through #mayInvoke/#admit. kill has the same gap.

Fix sketch: give stop and kill an author, and apply the same admission as invoking (owner, or anyone when the channel is shared). Hosts relaying for peers must pass the verified author, as they do for ask.

Dogfooding: #5